Neutronic device



-ABSTRACT THE DISCLOSURE A neutronic device which includes a tantalumelement having bonded thereto a U 235 enriched alloy layer of uraniumand nickel for use in supplying ionizing radiation inthermionicconverters, nuclear reactors, and the like.

This is a division of United States patent application Ser. No. 412,685,filed Nov. 20, 1964 and now Patent No. 3,367,02

This invention relates generally to neutronic devices of various types,such as ion chamber electrodes or fuel elements, which are used tosupply ionizing radiation in the form of high energy fission productsfor various applications, such as thermionic converters, nuclearreactors and the like. More particularly, this invention relates to amethod of providing a very thin, fissionable layer of a U 235 enriched,uranium-nickel alloy on a tantalum base in such devices.

In this specificatiom by the term U 235 enriched I refer to uranium inits pure state or as it may exist in a uranium-nickel alloy wherein theU 235 isotope content of the uranium is greater than about 50 atomicpercent, and, typically about 93 atomic percent with the balance beingsubstantially'the U 238 isotope. However, for the purposes of thepresent invention the U 235 and U 238 isotopes may be considered to havethe same metallurgical properties. Also by the term atomic percent Irefer to the proportion of atoms of one element to the total number ofatoms of all elements present in themass.

Zirconium is widely used as a base material in the construction ofneutronic devices, such as ion chamber electrodes and the like, due toits good machineability, high melting point, high structural strengthand low neutron cross section. In many of these neutronic devices, it isdesirable to provide a thin, U 235 enriched layer of uranium on azirconium base, whereby the uranium layer has suflicient thickness toproduce a maximum amount of fission fragment fiux, but yet the layer issufficiently thin so as not to produce excessive nuclear heating of thedevice. Theoretically, a layer of pure U 235 enriched, uranium having athickness of about 0.00037 inch or about 0.4 mil should satisfy theserequirements,

However, it is very difficult to thermally bond such ,a thin layer ofpure uranium directly on a zirconium base by melting a very thin foil ofthe uranium on the zirconium base due to the relatively poor wetting.qualities of molten uranium on zirconium and the resulting tendency ofthe molten uranium to bead up at various points on the zirconium base.Also, electrodeposition methods of providing a very, thin layerof pureuranium of less than one mil in thickness on a zirconium base aregenerally unsatisfactory, since .the uranium film deposited will readilyoxidize, particularly in an aqueous plating bath. Uranium oxide filmsare undesirable in neutron devices due to the dilutionof the uraniumconcentration which reduces the maximum fission fragment flux releasedper unit area.

Therefore, it is the principal object of the present invention toprovide a method of forming a thin, U 235 enriched, uranium alloy layeron a tantalum member or nited States Patent ice element in a neutronicdevice which will provide a maximum amount of fission fragment fluxwithout causing excessive nuclear heating of the device.

It is another object of the present invention to provide a very thin,fissionable, uranium-nickel alloy surface layer on a tantalum memberwhich is preferably bonded to a zirconium base member used in theconstruction of neu: tronic devices, such as ionization chamberelectrodes or thermionic converters and the like.

It is a further object of the present invention to provide a method offorming a very thin, fissionable, uraniumnickel alloy surface layer ontantalum which avoids the undesirable formation of uranium oxide.

It is a still further object of the present invention to provideneutronic devices, such as ion chamber electrodes and the like, having aU 235 enriched layer of a uraniumnickel alloy on a tantalum member orelement whereby the devices are capable of operating at comparativelylow temperatures but which will yield a high fission fragment These andother objects are accomplished in accordance with the present inventionby positioning a very thin nickel foil between a uranium foil ofsuitable thickness and a tantalum element and then heating the compositestructure at a suitable temperature under a vacuum or inert atmospherefor a sufiicient time so that the nickel and uranium will melt and forma thin, U 235 enriched, alloy layer of nickel and uranium which wets andadheres to the tantalum member. The tantalum member of the resultantcomposite structure may then be chemically bonded, brazed or otherwisesecured to any suitable structural backing or base material, such aszirconium, which is used in the construction of the particular neutronicdevice being fabricated. As will hereinafter be more fully explained,the thickness of the nickel and uranium foils which form the desiredalloy may vary in accordance with the present invention depending on thedesired operation conditions for the particular neutron device beingfabricated. These operating conditions include the required amount offission fragment flux and the desired operating temperature. Also, thepresence of the nickel in the uranium-nickel alloy layer improves theoxidation resistance of the uranium component.

Other objects, features and advantages of the present invention will beapparent from the following detailed description of certain embodimentsand specific examples thereof, especially when taken in conjunction withthe accompanying drawing in which FIGURE 1 is a crosssectional View of atypical ionization chamber electrode embodying the present invention.

As shown in the drawing, the electrode 10 consists of a flangedgenerally cylindrical base member 11 having a fiat end surface 12. Inthe embodiment of the present invention shown in the drawing, the basemember 11 preferably is made of zirconium, although it may also be madeof molybdenum or other high melting point metals or alloys having a lowneutron cross section which are suitable for use in fabricatingneutronic devices of this type. A strip or foil of tantalum 14, which ispreferably only a few mils in thickness, is bonded to the end surface 12of the zirconium base member by means of a nickel braze layer 16.However, it should be appreciated that other means, such as :a copperbraze, a chemical adhesive and the like, may be suitably employed tojoin the tantalum strip 14 to the metallic base member 11.

In accordance with the present invention, the outer surface layer 18 ofthe electrode 10 consists of a very thin, fissionable, U 235 enrichedlayer of a uranium-nickel all-oy which provides the desired amount ofionizing radiation or fission fragment flux. It should be understoodthat the thickness of the uranium-nickel alloy layer 18 is greatlyexaggerated in the drawing for the purpose of illustration, since thethickness of this layer is preferably less than about 2 mils, andideally about 0.4 mil. The tantalum strip or foil 14 and the nickelbraze layer 16 may be of any suitable thickness.

Tantalum has a melting point of about 2996 C. as compared to a meltingpoint of about 1445 C. for nickel and about 1133 C. for uranium. Also,nickel and uranium are miscible in all proportions in the liquid stateat temperatures above the melting point of nickel and in certainproportions at temperatures below the melting point of either nickel oruranium. For instance, a uranium-nickel alloy consisting of 33 atomicpercent nickel and 67 atomic percent uranium forms a eutectic mixturewhich may exist in the liquid state at a temperature of about 738 C.Moreover, nickel forms a low melting eutectic mixture with tantalum at atemperature of about 1360 C. and uranium will alloy with tantalum at itsmelting point of about 1133" C.

However, in accordance with the present invention, it is undesirable forthe uranium to diffuse into and alloy with tantalum to any appreciableextent, since this will dilute the maximum amount of fission fragemntflux released by the uranium per unit area. Also in accordance with thepresent invention it is undesirable for the nickel to diffuse into oralloy with tantalum to any appreciable extent, but rather, the nickelshould alloy with the uranium to provide a molten uranium-nickel alloywhich has good wetting properties on a tantalum surface.

Hence, in accordance with the subject process, a uranium-nickel alloylayer is formed on a tantalum element by placing a thin nickel orsuitable nickel base alloy foil between the tantalum member and a thin,U 235 enriched, uranium foil and heating the nickel and uranium foilsfor a sufiicient time at a temperature ranging between the lowereutectic melting temperature of nickel and uranium which is 738 C. andthe melting temperature of uranium of about 1133 C. to form a moltenuranium-nickel alloy which wets the desired surface area of the tantalumelement. Preferably the composite structure is heated to a temperatureranging between about 1000 C. to about 1100 C. However, when anappreciable amount of nickel, i.e., about 50 atomic percent is to be:alloyed with the uranium, it may be desirable or necessary to heat thecomposite structure up to temperatures of about 1500 0, provided that anappreciable amount of diffusion of the uranium or nickel into thetantalum surface is not caused at these higher temperatures.

This may be conveniently accomplished in accordance with the presentinvention by heating the nickel and uranium foils and the tantalumelement in a suitable apparatus, such as a electrical induction furnace,to a temperature within the aforementioned temperature range. However,this heating step should be carried out in a nonoxidizing environment,such as a vacuum or inert atmosphere of argon, neon or the like, toprevent any oxidation of the uranium foil which, as previouslymentioned, is undesirable. After the molten uranium-nickel alloy haswetted the desired surface area of the tantalum element, it is cooled ina vacuum or inert atmosphere so that it will solidify and become bondedto the tantalum base member. The nickel-uranium alloy thus formed istightly adherent to the tantalum element due to an adhesive-like bondingwith the tantalum surface.

The uranium content in the uranium-nickel alloy may vary considerably inaccordance with the present invention, depending on the thickness of theuranium and nickel foils which are employed, although the uraniumcontent normally should not be less than about 50 atomic percent of thealloy layer, and preferably not less than about 70 atomic percent of thealloy layer. Also, when uranium foil employed is relatively thick ascompared to the nickel foil used in forming this alloy layer, the alloylayer which is formed may not be homogeneous throughout but the nickelmay be alloyed with the uranium only adjacent the tantalum surface dueto the differences in diffusion rates of uranium and nickel upon beingmelted. In this instance,

. 4 the outermost portion of the alloy layer would be pure uranium. a a

By way of example, a nickel-uranium alloy layer was provided on atantalum foil which was bonded to a zirconium base member by a nickelbraze to form an ion chamber electrode similar to that shown in thedrawing by using the following procedure. A 0.75 milthick, U 235enriched, uranium foil, a 5 mil thick tantalum foil, two 0.1 mil thicknickel foils and a zirconium structural member were each cleaned in aconventional ultrasonic cleaning device three separate times utilizingthree different solutions which included trichloroethylene, acetone andmethyl alcohol. The zirconium base member was then degassed by firing itat a temperature of about 1200 C. under vacuum in a suitable electricalinduction furnace for about 30 minutes. The tantalum foil was similarlydegassed by vacuum firing at about 2000 C. for about 15 minutes. Theuranium foil was cleaned in a 50% concentrated aqueous nitric acidsolution followed by a water and methyl alcohol rinse in a conventionalultrasonic cleaning apparatus. Since uranium oxidizes very rapidly, thefoil was cleaned just prior to the brazing step. These cleaning anddegassing operations are, of course, desirable to remove any impuritiesfrom the materials.

The brazing step was then carried out in the following manner. One ofthe nickel foils was then sandwiched between the uranium foil and thetantalum foil and the composite structure was spot-Welded together byconventional means whereby an electrical current was passed through thestructure at various locations. Then. in accordance with the subjectprocess, the composite foil structure was placed in a suitable inductionheated vacuum furnace and heated to a temperature of about 1000 C. forabout 30 minutes, thereby causing the nickel and uranium to melt andform the desired uraniumnickel alloy which adhered to the surface of thetantalum foil upon subsequent cooling in a vacuum. After the compositestructure was cooled, another strip of nickel foil was sandwichedbetween the tantalum foil and the zirconium base member. The lattercomposite structure was next spot-welded together and subsequentlyheated under vacuum by an electrical induction heater to a temperatureof about 965 C. for about 30 minutes. Under these conditions the nickelbrazing foil fused with the zirconium causing the tantalum base andzirconium member to become bonded together upon subsequent cooling. Theresultant fissionable uranium-nickel alloy surface layer consisted ofabout 19.5 atomic percent nickel and about 80.5 atomic percent uranium.

In the above-described example of the present invention there was notendency of the uranium-nickel alloy to head up on the tantalum foilbase due to the good wetting properties of the molten alloy on tantalum.Also, the uranium was not deposited in the form of an oxide layer as inthe instance of conventional electrodeposition methods of forming thinuranium films on a metallic base.

Of course, it will be understood by those skilled in the art that theprocess conditions employed to form the resulting structure described inthe above example may be varied in accordance with the presentinvention, and the scope of the present invention is not intended to belimited thereby, except as defined by the following claims.

I claim:

1. A neutronic device comprising a tantalum member having a fissionablelayer of a uranium-nickel alloy nickel-brazed thereto.

2. A neutronic device comprising a tantalum member having a very thin,fissionable layer of a U 235 enriched uranium-nickel alloy nickel-brazedto said tantalum, said layer including not less than about 50 atomicpercent uranium.

3. The neutronic device of claim 2 wherein said layer has a thickness ofless than about 2 mils.

4. A neutronic device comprising a zirconium base 5 member, a relativelythin tantalum foil nickel-brazed to said base member, and a thin,fissionable layer of a uranium-nickel alloy bonded to said tantalumfoil, said uranium-nickel alloy layer including not less than about 70atomic percent of uranium.

5. An ion chamber electrode comprising a zirconium base member, a thintantalum foil bonded to said base member by a nickel braze layer, a thinfissionable layer of a uranium-nickel alloy bonded to said tantalumfoil, said uranium-nickel including not less than about 70 atomicpercent U 235 enriched uranium and having a thickness of about 0.4 mil.

Reactor Materials, vol. 8, N0. 4, Winter 1965-1966, page 231.

0 CARL D. QUARFORTH, Primary Examiner.

M. J. SCOLNICK, Assistant Examiner.

